This invention relates to a method of logging subsurface earth formations penetrated by a wellbore and more particularly to a method of identifying gas, oil and water zones in a subsurface formation having known acoustic characteristics.
Wellbores are conventionally logged to determine rock and fluid properties of subsurface formations that are traversed by the wellbore. Sonic logs, also referred to as acoustic logs, acoustic-velocity logs, and continuous velocity logs, are well logs of the travel time (transit time) for acoustic waves over a unit distance, and hence the reciprocal of the longitudinal wave (p-wave) velocity. Measurements are usually in microseconds per foot, though the log is normally displayed as velocity in feet per second versus depth. Such logs are used for porosity determinations as well as for furnishing geophysicists with velocity information for use in seismic interpretation. Other logs useful for determining porosity of subsurface formations are neutron logs and density logs.
In U.S. Pat. No. 4,131,875 there is described a method and apparatus for preferentially exciting and for extracting late arrivals at low frequencies in an acoustic investigation of a borehole. It is there pointed out that a transmitter spectrum whose frequencies extend with significant amplitudes down to about 500 Hz would be particularly desirable. Acoustic transmitters for producing sonic pulses containing low frequencies have been described in the art for investigating early arrivals, particularly in cased holes as is shown in U.S. Pat. No. 3,909,775. A low frequency transducer for generating acoustic energy above 75 db level for a bandwidth from 2.4 KHz to 9.6 KHz is described in U.S. Pat. No. 3,845,333.
There is published in THE OIL AND GAS JOURNAL, beginning with the May 15, 1978 issue, a series of articles entitled "Practical Log Analysis". In the fifteenth series, published in May 12, 1979 issue of THE OIL AND GAS JOURNAL, it is pointed out that basically three types of porosity logs are available: acoustic (sonic), neutrons, and density logging devices. As there noted, all porosity logs are primarily responsive to porosity but other formation characteristics influence the measurements. Various combinations of the three logging measurements can be used to determine specific lithologies, porosity, and, under certain circumstances, type and amount of fluid in the pore space.
The ninth series found in the Sept. 25, 1978 issue of THE OIL AND GAS JOURNAL, beginning at page 96, is entitled "Neutron Density Log As A Valuable Open Hole Porosity Tool". There it is said that the neutron density type logs are rapidly becoming the standard porosity device for open hole evaluation by wireline techniques. It is further pointed out that with the neutron density log one can obtain lithology, porosity, hydrocarbon types; locate shaley gas reservoirs; and serve as the basis for the advanced computer programs when combined with the basic resistivity devices.